1. Field of the Invention
The present invention relates generally to an optical waveguide device integrated module with a semiconductor laser and an optical waveguide device that are mounted on a submount and to a method of manufacturing the same.
2. Related Background Art
In the optical communication field, it is considered important to develop a hybrid integrated optical module including a semiconductor laser, an electronic element, an optical fiber, and the like that are integrated on a quartz-based lightwave circuit platform. This is an indispensable technique for reducing the size and cost of modules. In the technique, it is important to fix each element with high precision to minimize transfer-loss.
A surface mounting optical module has been proposed in which a semiconductor laser and a single mode fiber are bonded directly using a V-groove Si substrate (IEICE (The Institute of Electronics, Information and Communication Engineers) Conference 1997, C-3-63). FIG. 12 shows a structural view. Alignment keys 26 are formed in a Si substrate 24 and a semiconductor laser 25. The alignment keys 26 are subjected to image recognition, so that the center of a V-groove 27 and a position of an emission point of the semiconductor laser 25 are detected. Thus, position adjustment is carried out with high precision. Mounting variations of about xc2x10.61 xcexcm in the x direction and about xc2x11 xcexcm in the z direction are achieved with respect to the V-groove 27 of the Si substrate 24. An optical fiber 28 is mounted in the V-groove 27 accurately. The V-groove 27 is formed with high precision by anisotropic etching of Si. Similarly, the optical fiber 28 is produced with its outer dimension and core center controlled with high precision. Therefore, the fiber 28 is fitted into and is fixed to the V-groove 27, so that the optical fiber 28 can be fixed with respect to the semiconductor laser 25 with high precision.
On the other hand, in order to achieve the increases in density of optical disks and in definition of a display, a small short-wavelength light source is required. Techniques for obtaining short wavelength light include blue light generation using a semiconductor laser and an optical waveguide second harmonic generation (hereinafter referred to as xe2x80x9cSHGxe2x80x9d) device employing a quasi-phase-matched (hereinafter referred to as xe2x80x9cQPMxe2x80x9d) system (Yamamoto et al., Optics Letters Vol. 16, No. 15, p1156, (1991)).
FIG. 13 shows a schematic structural view of a blue light source using an optical waveguide QPM-SHG device. A wavelength variable semiconductor laser having a distributed Bragg reflector (hereinafter referred to as xe2x80x9cDBRxe2x80x9d) region (hereinafter referred to as a xe2x80x9cwavelength-variable DBR semiconductor laserxe2x80x9d) is used as a semiconductor laser. Numeral 29 is a 100-mW class AlGaAs-based wavelength-variable DBR semiconductor laser in a 0.85-xcexcm range. The semiconductor laser includes an active layer region and a DBR region. An amount of current applied to the DBR region is varied, so that the emission wavelength can be varied.
An optical waveguide QPM-SHG device 30 as a wavelength conversion device includes an optical waveguide and a region whose polarization is reversed periodically, which are formed on a x-cut Mg-doped LiNbO3 substrate. A SiO2 protective film 31 is formed on the surface at which the optical waveguide is formed. The wavelength-variable DBR semiconductor laser 29 and the optical waveguide QPM-SHG device 30 are fixed so that the active layer and the surface at which the optical waveguide is formed are in contact with a submount 32, respectively (hereinafter referred to as xe2x80x9cface down mountingxe2x80x9d). A laser beam obtained from an emission surface (from which a beam leaves the laser 29) of the wavelength-variable DBR semiconductor laser 29 is coupled directly to the optical waveguide of the optical waveguide QPM-SHG device 30.
The optical coupling adjustment is carried out with the semiconductor laser emitting a beam, and with respect to a 100-mW laser output, a 60-mW laser beam was coupled to the optical waveguide. The amount of current applied to the DBR region of the wavelength-variable DBR semiconductor laser is controlled and thus the emission wavelength is set within a tolerance of the phase matched wavelength of the optical waveguide QPM-SHG device. Currently, about 10-mW blue light with a wavelength of 425 nm has been obtained.
In an optical module in which a semiconductor laser and an optical fiber are integrated, the optical fiber is mounted in a V-groove formed in a Si submount and the semiconductor laser is mounted using the V-groove as a reference position. The optical fiber has a cylindrical shape and has a core portion (an optical propagation region) formed in its center. The optical fiber is formed with its diameter controlled with high precision. In addition, the V-groove in the Si submount also is formed with high precision using the anisotropic etching of Si. Therefore, the optical fiber is mounted with its core portion as the center of the optical fiber being adjusted with respect to the Si submount with high precision. On the other hand, the alignment keys used for positioning the semiconductor laser also are formed in reference to the V-groove and therefore the semiconductor laser also can be mounted with high accuracy.
In a planar optical waveguide device with an optical waveguide formed on a surface of a LiNbO3 substrate by proton exchange or Ti diffusion (devices other than optical waveguide devices with an optical waveguide layer (core) in the coaxial center like an optical fiber are referred to as xe2x80x9cplanar optical waveguide devicesxe2x80x9d in the present invention), the distance from the substrate surface to the optical waveguide is controlled with high precision. In an integrated module including a semiconductor laser and a planar optical waveguide device, the semiconductor laser is fixed with a solder material and the optical waveguide device is fixed with an adhesive by face down mounting. In the semiconductor laser, generally, an active layer is formed on an n-type substrate, and a P-type clad layer and further a p-side electrode are formed thereon. Therefore, the distance from the p-side surface to the active layer is about 3 xcexcm. The solder material has a thickness of about 1 to 2 xcexcm. Consequently, the distance from the submount to the active layer after mounting is about 4 to 5 xcexcm. This distance can be controlled to be about xc2x10.2 xcexcm through the adjustment of the amount of pressure applied to the semiconductor laser during the mounting.
On the other hand, since the optical waveguide portion of the planar optical waveguide device is formed at the substrate surface, the distance from the substrate to the optical waveguide portion is about 1 xcexcm. Therefore, there is a difference in level of about 3 to 4 xcexcm between the active layer of the semiconductor laser and the optical waveguide portion of the optical waveguide device. Consequently, it has been difficult to carry out the adjustment without allowing the semiconductor laser to emit a beam (hereinafter referred to as xe2x80x9cpassive alignment mountingxe2x80x9d).
A method has been proposed in which a thick film is formed on a planar optical waveguide device to allow the levels of an active layer of a semiconductor laser and an optical waveguide portion of the planar optical waveguide device to coincide with each other. This method, however, has the following problems.
(1) Conditions for manufacturing the optical waveguide vary due to the increase in temperature of a substrate during the formation of the thick film. Particularly, in the case of a SHG device employing the QPM system, a phase matched wavelength may vary and the wavelength conversion characteristics may deteriorate with the variation in refractive index of the optical waveguide.
(2) After being formed, the thick film shrinks and therefore, a substrate may warp. The warping makes it difficult to mount the device on a submount.
(3) The thick film has a thickness of about several micrometers. Therefore, it is difficult to control the thickness to be uniform.
(4) In fixing the optical waveguide device with an adhesive, when the thickness of the adhesive is not uniform, heat from the submount cannot be conducted uniformly. Therefore, particularly in the wavelength conversion device employing the QPM system, the phase matched wavelength may vary or the wavelength conversion characteristics may deteriorate.
On the other hand, in the adjustment of optical coupling between the semiconductor laser and the planar optical waveguide device in the width direction, conventionally, an adhesive was applied after the optical coupling adjustment and then was dried to fix them. Therefore, misalignment after the adjustment might be caused by the stress exerted during the application of the adhesive or the shrinkage of the adhesive upon curing.
In the optical waveguide QPM-SHG device utilizing the second harmonic generation, the power of harmonic light obtained is proportional to the square of the power of a fundamental wave to be coupled. Therefore, it is indispensable to improve the coupling efficiency and reduce the variations among samples.
Therefore, it is an object of the present invention to solve the above-mentioned problems and to provide an optical waveguide device integrated module in which a semiconductor laser and a planar optical waveguide device are mounted with their positions in their height direction controlled with high precision and to provide a mounting method for manufacturing the same.
In order to achieve the aforementioned object, an optical waveguide device integrated module according to the present invention includes a semiconductor laser and an optical waveguide device on a submount. The optical waveguide device includes an optical waveguide formed on a surface of its substrate. The semiconductor laser and the optical waveguide device are mounted on the submount with both a surface of the semiconductor laser at which an active layer is formed and a surface of the optical waveguide device at which the optical waveguide is formed facing the submount. The submount is combined with the semiconductor laser or the optical waveguide device to form one body using an adhesive with a spacer being interposed therebetween. The spacer maintains a substantially uniform distance between the submount and the semiconductor laser or the optical waveguide device.
A mounting method for manufacturing an optical waveguide device integrated module according to the present invention is directed to a mounting method for manufacturing an optical waveguide device integrated module including a semiconductor laser and an optical waveguide device mounted on a submount with both a surface of the semiconductor laser at which an active layer is formed and a surface of the optical waveguide device at which an optical waveguide is formed facing the submount. The method includes mounting at least one of the semiconductor laser and the optical waveguide device on the submount with an adhesive, with a spacer between the submount and the semiconductor laser or the optical waveguide device. The spacer maintains a substantially uniform distance between the submount and the semiconductor laser or the optical waveguide device.